JP2008173717A - Electric discharge machining apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the effect of the stray capacity of the cable of a voltage measuring circuit upon the surface roughness of a machined surface in finish machining. <P>SOLUTION: Voltage between electrodes is measured by connecting a voltage measuring circuit 4 between an electrode 1 and the electrode of a workpiece 2 by means of a parallel reciprocation cable 3 via a resistor R. The parallel reciprocation cable 3 is formed such that two lead wires are so fixed as to be in parallel with each other at a constant interval with insulating and flexible materials, such as rubber and resin, like the feeder wire for a TV antenna. The stray capacitance of the parallel reciprocation cable is reduced by widening the interval between the two lead wires of the cable. As a result, the high frequency voltage in finish machining can be measured by the voltage measuring circuit without being attenuated. Further, the pulsation of the voltage to be input to the voltage measuring circuit can be suppressed because the resistor R is inserted therein. Because of the reduction of the stray capacitance, the electric discharge energy in the finish machining can be suppressed to be small, and the roughness of the machined surface can be improved. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electric discharge machining apparatus capable of improving the surface roughness of a machined surface in finishing.

In electric discharge machining, a voltage is applied between the electrode in the machining fluid and the workpiece to generate arc discharge. At the same time as the workpiece melts due to the heat of this discharge, the machining fluid is heated rapidly, causing a vaporization explosion and blowing off the melted workpiece. By repeating this frequently, processing proceeds. Further, since small discharge traces formed by discharge gather to form a processed surface, the size of each discharge trace determines the surface roughness.
Since the size of the discharge trace generally depends on the size of the individual discharge energy, it is necessary to suppress the single-shot discharge energy as small as possible in order to improve the surface roughness in finishing and the like.

In general, a capacitor component called stray capacitance exists between electrically insulated conductors. Since this stray capacitance exists also between the electrode and the workpiece (between the electrodes), this stray capacitance is inevitably charged when a voltage is applied between the electrodes. Therefore, when discharge occurs, not only the energy supplied from the machining power supply but also the energy stored in this stray capacitance is supplied between the electrodes. In finishing and the like, if the energy supplied from the machining power source is reduced in order to keep the discharge energy small, the energy released from the stray capacitance between the electrodes has a great influence on the single discharge energy. This affects the surface roughness of the processed surface.
The main stray capacitance exists in the rough machining power source, the rough machining cable, between the work table and the electrode, and between the electrodes.

Therefore, insert a switch between the poles of the roughing cable and turn it off during finishing to disconnect the roughing power supply or stray capacitance of the roughing cable, or to insulate and fix the workpiece from the table Thus, an invention in which stray capacitance between the table and the electrode is separated is known (see Patent Document 1).
Although a large stray capacitance is eliminated by the method according to the invention described in Patent Document 1, a stray capacitance also exists in a voltage measurement circuit that measures an interelectrode voltage between an electrode and a workpiece.

  In general, an electric discharge machining apparatus recognizes the discharge state and controls the relative feed of the electrode to the workpiece, and measures the voltage between the electrode being machined and the workpiece, and based on the measurement result, the electrode Controlling the relative feed is generally performed, but stray capacitance also exists in the measurement cable connecting the gap and the control circuit and the measurement circuit itself. As a measure to prevent this stray capacitance from affecting the surface roughness of the finishing process, the inter-electrode voltage is optically insulated with a photocoupler, etc., and then transmitted to the control circuit to reduce the stray capacitance and finish processing. A method for improving the surface roughness is proposed (see Patent Document 2).

  Also, an invention is known in which a resistor is inserted in series between the electrodes for detecting the voltage between the electrode and the workpiece, and the surface roughness is improved by suppressing discharge due to the stray capacitance by this resistor ( (See Patent Document 3).

JP 2002-66843 A JP-A-7-328844 JP 2000-42835 A

  In finishing processing, it is necessary to reduce the discharge energy in order to improve the surface roughness of the processed surface, but it is necessary to use a voltage measurement circuit that measures the voltage between the workpiece and the electrode and a cable that connects the electrode. The discharge due to the existing stray capacitance is added to the normal intended discharge, increasing the discharge energy and causing the surface roughness of the processed surface to decrease. As a method for reducing the influence of stray capacitance existing in the cable of the voltage measurement circuit for measuring the voltage between the electrodes, a method using a photocoupler as disclosed in Patent Document 2 is theoretically possible. However, it is anticipated that it will be quite difficult to accurately insulate and transmit the high-frequency voltage during finishing, and even if it can be realized, it will be very expensive.

In addition, as described in Patent Document 2, a method of detecting a voltage between electrodes by inserting a resistor into a cable of a voltage measurement circuit between electrodes, and suppressing the discharge of stray capacitance, a measured waveform of the voltage measured between the electrodes. There is a problem that becomes dull. In general, coaxial cables and shielded wires are used as voltage measurement lines for measuring the voltage between the workpiece and the electrode. Since such cables have a large line capacitance, the above-mentioned series resistance and cable capacitance are used. There is a problem that a low-pass filter is formed and the measurement waveform becomes dull.
FIG. 5 is a diagram illustrating a measurement voltage when a resistance of 200Ω is inserted into a cable of a voltage measurement circuit for measuring a voltage between electrodes, a high-frequency voltage is applied between the electrodes, and the voltage between the electrodes is measured. Time and the vertical axis represent voltage. FIG. 5A shows the voltage between the workpiece and the electrode, and FIG. 5B shows the input voltage of the voltage measuring circuit. In this example, the input voltage of the voltage measurement circuit is reduced by about 35%.

  Accordingly, an object of the present invention is to reduce the influence on the voltage measurement accuracy of the voltage measurement circuit for measuring the voltage between the electrodes and reduce the influence on the surface roughness due to the stray capacitance of the cable of the voltage measurement circuit. An electrical discharge machining apparatus is provided.

  An electric discharge machining apparatus having a voltage measurement circuit for measuring an interelectrode voltage between an electrode being processed and a workpiece, and controlling a discharge state and feeding of an electrode based on a measurement result by the voltage measurement circuit. The invention provides two measurement lines, a measurement line connecting the voltage measurement circuit and the electrode, and a measurement line connecting the voltage measurement circuit and the workpiece, with an insulating and flexible material and a radius of the measurement line. In addition to being fixed in parallel with a distance of 4 times or more, a resistor is inserted between the electrodes, and this configuration reduces the stray capacitance of the measurement line, thereby reducing the stray capacitance. This reduces the discharge energy caused by the surface to improve the surface roughness of the machined surface during finishing, reduces the attenuation of the interelectrode voltage input to the voltage measurement circuit, and suppresses vibration. is there.

  According to the second aspect of the present invention, an RC parallel circuit in which a resistor and a capacitor are connected in parallel is connected in series, and a resistor-capacitance voltage divider that divides and outputs a voltage between the electrodes is connected between the electrodes. The voltage between the electrodes is measured by the voltage measurement circuit via the voltage divider, and the capacitance of the RC parallel circuit connected to the voltage measurement circuit of the resistance-capacitance voltage divider is the stray capacitance that the measurement cable of the voltage measurement circuit has. With this configuration, the discharge energy due to the stray capacitance is reduced, the attenuation of the interelectrode voltage input to the voltage measurement circuit is reduced, and the occurrence of vibration is also suppressed. Further, in the invention according to claim 3, when the resistance of one RC parallel circuit of the resistive voltage divider is R1, the capacitance of the capacitor is C1, the resistance of the other parallel circuit is R2, and the stray capacitance is C2, The capacitance C1 of the capacitor was determined by the voltage division ratio and the stray capacitance C2 of the measurement cable so that R1 · C1 = R2 · C2.

  The discharge energy due to the stray capacitance can be reduced, and the surface roughness of the machined surface during finishing can be improved. Further, it is possible to prevent the measurement voltage input to the voltage measurement circuit from being attenuated, and to suppress the vibration of the measurement voltage.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram of a main part of a first embodiment of the present invention. This first embodiment is characterized in that a voltage measuring circuit for measuring an interelectrode voltage between an electrode and a workpiece is connected using a parallel reciprocating cable having a wide line distance, such as a feeder line for a television antenna. Is.
Also in FIG. 1, reference numeral 1 is an electrode, and reference numeral 2 is a workpiece of a workpiece. A voltage is applied between the electrode 1 and the work 2 from a machining power source (not shown). Reference numeral 4 denotes a voltage measuring circuit for measuring the voltage between the electrode 1 and the work 2. The discharge state is monitored by the voltage measured by the voltage measuring circuit 4, and the electrode 1 with respect to the work 2 is monitored. The relative feed speed is controlled, and the voltage application on time and off time are controlled.

  In this voltage measuring circuit 4, a measuring wire for measuring the voltage between the electrodes 1 and 2 is connected to both sides between the electrodes 1 and 2. The cable of the conducting wire of the measuring wire is constituted by a parallel reciprocating cable 3 in which two conducting wires have a wide distance between the wires, like a feeder wire for a television antenna. That is, the two conducting wires are fixed with an insulating and flexible material such as rubber or resin so that the distance between the wires is parallel. Furthermore, the conducting wire connected to the electrode 1 side is connected with a resistor R arranged in series. The stray capacitance C of the parallel reciprocating cable 3 is reduced by increasing the distance between the parallel reciprocating cables 3. Since the stray capacitance C of the parallel reciprocating cable 3 is reduced, the effect of the low-pass filter is small even if the resistor R is inserted, and the high-frequency voltage applied during finishing processing is transmitted without being attenuated and can be measured by the voltage measuring circuit 4. I am doing so. Thereby, the voltage between the electrodes 1 and the workpiece 2 can be accurately measured.

  In addition, since the stray capacitance becomes small, even when the discharge is generated between the electrode 1 and the work 2, even if the energy stored in the stray capacitance is discharged, the discharge energy becomes small and the discharge given by the stray capacitance. As a result, the discharge energy in the finishing process can be reduced, and the surface roughness of the finished surface can be improved.

  FIG. 2 is an explanatory diagram of the electrostatic capacitance (floating capacitance) C and the inductance L of the parallel reciprocating cable 3. When the conducting wires of the pair of measurement wires constituting the parallel reciprocating cable 3 are 3a and 3b, the radius of the conducting wires 3a and 3b is a, and the distance between the central axes of the conducting wires 3a and 3b arranged in parallel is d. The electrostatic capacitance (floating capacitance) C per unit length is expressed by the following equation (1). The inductance L is expressed by equation (2).

C = πε / log (d / a) (1)
L = (μ / π) × log (d / a) (2)
Here, ε is the inductivity of the insulator between the conductors, and μ is the magnetic permeability of the space.

  As is clear from the equation (1), the electrostatic capacitance (floating capacitance) C decreases as (d / a) increases. That is, the capacitance (floating capacitance) C can be reduced by increasing the distance d between the central axes of the conducting wires 3a and 3b. On the other hand, the inductance L increases as the distance d between the central axes of the conducting wires 3a and 3b increases, and the electrostatic capacitance (stray capacitance) C and the inductance L are in a close relationship.

  If the capacitance (floating capacitance) C is determined by deciding the distance d between the conductors 3a and 3b, fixing the fixed distance, and arranging them in parallel, with the main objective of reducing the capacitance (floating capacitance) C. , (2), inductance L is uniquely determined. That is, if the upper limit of the target electrostatic capacitance (stray capacitance) C is set, the lower limit of the line-to-line distance d is determined, and the lower limit of the inductance L is naturally determined.

  The capacitance per unit length in a general signal transmission coaxial cable is about 100 pF, but if the above (d / a) is set to about 4, the capacitance (floating capacitance) C is reduced to half or less. Can be suppressed. However, as (d / a) is increased, the electrostatic capacitance (floating capacitance) C can be reduced. However, since its effect is rapidly reduced, (d / a) <50 is a practical range.

  Further, as described above, when the line distance d is increased, the inductance L of the parallel reciprocating cable 3 tends to increase, and resonance easily occurs between the inductance L and the electrostatic capacitance (floating capacitance) C. The measurement waveform may vibrate.

  FIG. 6 shows a state in which the resistance R is eliminated and the voltage measuring circuit 4 is connected by the parallel reciprocating cable 3 between the electrode 1 and the work 2 in FIG. It is a figure which shows the voltage (FIG. 6 (a)) between electrodes when a high frequency voltage is applied, and the input voltage (FIG. 6 (b)) of the voltage measurement circuit 4. FIG. The horizontal axis represents time, and the vertical axis represents voltage. As shown in FIG. 6A, the inter-electrode voltage is not attenuated, but the input voltage of the voltage measuring circuit shown in FIG. 6B is oscillated.

  Therefore, in the first embodiment, as shown in FIG. 1, the voltage measuring circuit 4 for measuring the interelectrode voltage between the electrode 1 and the work 2 is connected to the electrode 1 and the work with a parallel reciprocating cable 3 via a resistor R. 2 is connected, and d / a (= inter-wire distance / conductor radius) is set to four times or more to suppress the vibration of the voltage input to the voltage measuring circuit 4.

  FIG. 4 is a diagram showing the interelectrode voltage (FIG. 4A) between the electrode 1 and the workpiece 2 measured in the first embodiment and the input voltage (FIG. 4B) of the voltage measurement circuit. . The horizontal axis represents time, and the vertical axis represents voltage. Compared with FIG. 5, the attenuation of the input voltage of the voltage measurement circuit shown in FIG. 4B is small. Since the inter-line distance d is increased, the electrostatic capacity (floating capacity) C of the parallel reciprocating cable 3 is reduced, and the electrostatic capacity (floating capacity) C of the parallel reciprocating cable 3 is small, so that a resistor R is inserted. Even so, the effect of the low-pass filter is small, and it has been shown that high-frequency signals at the time of finishing can be transmitted without being attenuated. Further, as described above, by setting the distance d between the parallel reciprocating cables 3 and the radius a ratio (d / a) of the conducting wire to 4 or more and less than 50 (4 ≦ (d / a) <50). In addition to minimizing the increase in inductance L, the resistor R is inserted in series to suppress the oscillation of the voltage input to the voltage measuring circuit 4.

  If the resistance R to be inserted is only to suppress vibration due to the inductance L of the parallel reciprocating cable 3, the resistance may be on either side of the parallel reciprocating cable 3, but the parallel reciprocating cable 3 and the voltage measurement circuit In order to prevent the discharge current of this capacitance from flowing between the electrodes, for example, as shown in FIG. 1, for example, if the work 2 is grounded, The resistor R needs to be inserted on the electrode 1 side. In addition, when the workpiece 2 is not grounded and the electrode 1 and the workpiece 2 are both lifted and processed, it is necessary to insert resistors on both the electrode 1 side and the workpiece 2 side.

  In addition, a parallel reciprocating cable 3 in which conductive wires are connected in parallel with a certain width using a material such as rubber or resin that is insulative and flexible, such as a feeder wire for a television antenna, has been conventionally used. Compared to conventional shielded wires and coaxial wires, there is a problem that the noise resistance is low. However, if it is finished, the generated noise is reduced and no practical problem occurs. If the influence of noise appears in roughing, the measurement cable may be switched between roughing and finishing. Moreover, if the cable is twisted and mounted, the influence of the magnetic field due to noise can be canceled.

  Note that “parallel” in the first embodiment is intended to reduce inductance, and does not mean geometric “parallel”, but may be “parallel” macroscopically. A sufficient effect can be obtained.

FIG. 3 is a schematic diagram of the second embodiment of the present invention. This second embodiment is a method of inserting a resistive capacitance divider 5 between the poles of a measurement cable. One capacitance of the resistive capacitance divider is used as a stray capacitance C2 included in the measurement cable 6, and the resistive capacitance is divided. The voltage divider 5 is configured.
The resistor-capacitance voltage divider 5 is an RC parallel circuit in which a resistor and a capacitor are connected in parallel, connected in series, connected to both ends of the voltage to be measured, and divided to extract the voltage. By aligning the constants (R1 · C1 = R2 · C2), the frequency characteristic becomes flat and the voltage division ratio can be made constant from DC to high frequency. In the second embodiment, one end of an RC parallel circuit composed of a resistor R1 and a capacitor C1 is connected to the electrode 1 side, and the stray capacitance C2 included in the measurement cable 6 of the resistor R2 and the voltage measurement circuit 4 is provided. The RC parallel circuit connected in parallel is connected to the RC parallel circuit of the resistor R1 and the capacitor C1, and the other end is connected to the work 2. That is, the capacitance of one of the RC parallel circuits constituting the resistive capacitance divider 5 is the resistive capacitance divider 5 constituted by the stray capacitance C2 included in the measurement cable 6 of the voltage measurement circuit 4, and the resistive capacitance divider 5 is Thus, the voltage between the electrodes is measured by the voltage measuring circuit 4.

  Then, the capacitance C1 of the capacitor is determined in accordance with the voltage dividing ratio by the resistors R1 and R2 and the stray capacitance C2 so that R1 · C1 = R2 · C2. As a result, the frequency characteristics become flat, the voltage division ratio is constant from direct current to high frequency, and the measurement waveform does not become dull even if the stray capacitance C2 of the measurement cable 6 is large. In addition, since resistance enters the voltage measurement circuit 4, the stray capacitance C2 of the measurement cable 6 is not discharged between the electrodes and the surface roughness is not deteriorated, and the resistance capacitance divider 5 is connected between the electrode 1 and the workpiece 2. Even if the measurement cable 6 between the electrodes is somewhat longer, the vibration of the voltage waveform input to the voltage measurement circuit 4 can be suppressed.

It is a principal part schematic diagram of the 1st Embodiment of this invention. It is explanatory drawing explaining the electrostatic capacitance (stray capacitance) and inductance which a parallel reciprocating cable has. It is a principal part schematic diagram of the 2nd Embodiment of this invention. In the 1st Embodiment of this invention, it is a figure which shows the voltage between electrodes when a high frequency voltage is applied between electrodes, and the input voltage of a voltage measurement circuit. It is a figure which shows the input voltage of the voltage between electrodes which measured by inserting resistance into the cable of the conventional voltage measurement circuit, and applying a high frequency voltage between electrodes, and a voltage measurement circuit. It is a figure which shows the voltage between electrodes measured when the high frequency voltage was applied between the poles in the state which connected the voltage measurement circuit between the poles with the parallel reciprocating cable, and the input voltage of the voltage measurement circuit.

Explanation of symbols

1 Electrode 2 Workpiece 3 Parallel Reciprocating Cable 4 Voltage Measurement Circuit 5 Resistance Capacitance Voltage Divider 6 Measurement Cable

Claims (3)

In an electric discharge machining apparatus that has a voltage measurement circuit that measures the voltage between the electrode and the workpiece being processed, and controls the discharge state and the feed of the electrode based on the measurement result by the voltage measurement circuit,
Two measurement lines, a measurement line connecting the voltage measurement circuit and the electrode and a measurement line connecting the voltage measurement circuit and the workpiece, are made of an insulating and flexible material and are four times the radius of the measurement line. An electric discharge machining apparatus characterized by being fixed and arranged in parallel with a distance therebetween, and a resistor inserted between the electrodes. In an electric discharge machining apparatus that has a voltage measurement circuit that measures the voltage between the electrode and the workpiece being processed, and controls the discharge state and the feed of the electrode based on the measurement result by the voltage measurement circuit,
An RC parallel circuit in which a resistor and a capacitor are connected in parallel is connected in series, and a resistor-capacitance voltage divider that divides and outputs an inter-electrode voltage is connected between the electrodes, and the inter-electrode voltage is supplied via the resistor-capacitor voltage divider. Discharge characterized in that the capacitance of the RC parallel circuit measured by the voltage measurement circuit and connected to the voltage measurement circuit of the resistance-capacitance voltage divider is the stray capacitance of the measurement cable of the voltage measurement circuit Processing equipment. When the resistance of one RC parallel circuit of the resistor-capacitance voltage divider is R1, the capacitance of the capacitor is C1, the resistance of the other parallel circuit is R2, and the stray capacitance is C2, R1 · C1 = R2 · C2
The electric discharge machining apparatus according to claim 2, wherein the capacitance C1 of the capacitor is determined by the voltage division ratio and the stray capacitance C2 of the measurement cable.

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